Nearly 20%of the US population suffers from autoimmune diseases, which are among the top ten causes of death by disease for women under 65 and the 4th leading cause of disability. This also imposes a severe economic burden measured in billions ofdollars. Therapies for many autoimmune disorders rely on gluco- corticoids (GC)to inhibit the production of cytokines, such as interferons (IFN)a/ (J,which are intimately linked to the pathogenesis of lupus, autoimmune diabetes and thyroiditis. Yet, serious side effects associated with GC use can outweigh the benefits of these otherwise effective and affordable drugs. Our long-term goal is to identify the key viewers and mechanisms of GC-mediated immunomodulation in health and disease, which is essential for developing more specific therapiesfor autoimmunity. GC signal through a ligand-dependent GC receptor, GR, which recruits accessory cofactors to regulate transcription. Specifically, GR-dependent repression of AP1- and NFxB-responsive genes accounts for certain anti-inflammatory activities of GC. While additional regulators must mediate the profound GC-induced immunosuyyression, their identity and mechanisms ofaction are obscure. We established a nuclear receptor cofactor, GR-interacting protein (GRIP)l as a corepressor for GR:NFKB and GR: API repression complexes. Unexpectedly, a yeast 2-hybrid screen for GRIP1 Repression Domain interactors yielded Interferon Regulatory Factor (IRF)3, a key effector of innate immune responses downstream ofToll- like receptors (TLR)3/4 and an essential transactivator for IFNp. Theobjective ofthis application is to decipher the functional interaction between GC and IRF3. Wehypothesize that GRIP1 is an IRF3 cofactor with a role in IFN production and autoimmunity. Wefurther propose that GRand IRF3 competefor GRIP1 such that hormone- activated GR,in addition to directly repressing APl/NFicB,sequesters GRIP1 away from IRF3;the resulting inhibition of IRF3-dependent transcription is potentially a critical yet unexplored component of GC immuno- suppression. Conversely, in response to bacterial or viral TLR3/4 ligands, IRF3 not only enhances cytokine transcription directly, but also depletes GRIP1 from GR:AP1and GR:NFx:B repression complexes thereby promoting an effective immune response. Our Specific Aims are to: (1) dissect the GRIP1:IRF3 interaction in vitro, in cells and in IRF3 complexes bound at the IFN-Stimulated Response Elements;(2)corroborate the relevance of this interaction by examining the cross-talk between GRand IRF3 in primary macrophages;(3) identify the role of GRIP1 in IRF3 transcription and,using GRIP1KO mice, in innate immune responses to virus, or to dsRNA and bacterial LPS in vivo. The GRIP1:IRF3interaction uncovered in an unbiased screen may have a broad physiological significance. We will explore a role of GRIP1in the immune system and decipher associated transcriptional mechanisms operative at IRF3 target genes. This work should provide a molecular basis for a functional antagonism between GC and IFN, and help define IRF3 as a novel target for GR- mediated immunosuppression, ultimately aiding in the design of more specific drugs for autoimmunity.